Modeling particle transport by bubbles for performance guidelines in airlift fermentors

A calculation method has been developed to model the statistical transport of biological particles in bubble-driven flows, with special reference to the biokinetics of environmental excursions experienced by individual cells, aggregated cells, or immobilization beads in airlift bioreactors. Interim developments on modeling the transport of such particles in concentric tube devices are reported. The calculation is driven by user-prescribed global parameters for the bioreactor geometry, bulk air flow rate, and particle parameters (size and slip speed). The algorithm calls on empirical data correlations for void fraction, bulk liquid flow rate, and bubble sizes and slip speeds, optimally selected from a large bibliographic database. The Monte Carlo algorithm concentrates on simulating particle transport in the bubbly riser flows.The packaged family of correlations and calculations represents, in effect, an expert system augmented by a transport simulation suited to characterizing the biokinetic response of cells cultured in airlift bioreactors.     

Scaling oxygen mass transfer in agitated fermentors

There are many scaling formulas that predict the oxygen mass transfer coefficient as k(L).a = constant.(Hp/V)(alpha)Vs(beta) Exponents alpha and beta frequently are scale dependent themselves. A general formula has been derived from the work of Calderbank,(1) Miller,(2) and Tilton,(3) resulting in k(L).a = C(1) phi + C(2) log (Pm/V) phi where phi equals the gas-holdup fraction and Pm/V equals the effective mechanical power input per unit of volume. This formula is consistent with the formula of Westerterp(4) modified by Miller.(2) Gas holdup can be predicted in several ways. Gas-sparged isothermal expansion power input, used for predicting phi, demonstrates that scaling can be done by using either superficial air velocity or volume per volume per minute for aeration.The importance of mixing in replenishing oxygen at the boundary layers of microorganisms will be assessed and compared with the k(L).a as the oxygen transfer ratelimiting step.     

Mixing and oxygen transfer in conventional stirred fermentors

A set of experiments has been performed in an industrial 112 m(3) fermentor in order to get a complete map of oxygen concentration and temperature distribution in the system. Five fermentations of non-Newtonian broths of two different strains, in various operating conditions, were examined. A simple model has been developed which takes into account both the mixing and the mass-transfer properties of the fermentor, and a dimensionless parameter has been identified which is sufficient to characterize the oxygen axial distribution in the reactor in any operating condition.     

Region-dependent oxygen transfer rate in the rectangular airlift reactor

Gas hold-up and the oxygen transfer in the zones of the internal loop airlift reactor with rectangular cross-section was studied. It was found, that the downcomer to the riser gas hold-up ratio depends on the gas flow rate, the physicochemical properties of the system and on the reactor height. The ratio of the downcomer mass transfer coefficient to the global mass transfer coefficient was less than 6%. The ratio of the downcomer to the global mass transfer coefficient slightly increased with increase of the gas flow rate and decreased with increase of the liquid viscosity. The proposed correlation for the global overall mass transfer coefficient predicts the experimental data well within 16.6% deviation. It was confirmed that the reactor height is the important parameter for a design and a scale-up of the airlift reactors.     

Gas holdup and overall volumetric oxygen transfer coefficient in airlift contactors

The two major types of airlift contactors, concentric-tube and external-loop, were investigated for their gas holdup (riser and downcomer) and overall mass transfer characteristics. Results obtained in batch charges of tap water and 0.15 kmol/m(3) NaCl solution are reported for external-loop airlift contactors having downcomer-to-riser cross-sectional area ratios, A(d)/A(r), ranging from 0.11 相似文献   

Experimental simulation of dissolved oxygen fluctuations in large fermentors: effect on Streptomyces clavuligerus

An experimental Monte Carlo method was used to study the effect of fluctuations in oxygen concentration on the synthesis of antibiotics by Streptomyces clavuligerus. Air was supplied to the culture in a 2-L fermentor in random cycles following the lognormal distribution in order to model the circulation within large production-scale vessels. Each cycle consisted of air supply for 5 s followed by no aeration for the balance of the cycle time which ranged from 8 to 44 s, with a mean time of 20 s. Comparable experiments were also conducted with constant period cycling of air and with continuous supply of air. The yields of cephamycin C and its precursor, penicillin N, were suppressed by the Monte Carlo simulation of circulation in a large tank, as compared to constant period cycling. The concentration of dissolved oxygen remained at a low, ca. 5% of saturation, for 5-10 h longer during the Monte Carlo experiment than during the periodic aeration experiment. The biosynthetic enzymes, which are sensitive to oxygen levels, were likely affected not only by the mean time of cycling but also by the distribution of the cycles.     

Modeling high-biomass-density cell recycle fermentors

Since intrinsic models, which take into account cell volume fraction, follow from proper application of the law of conservation of mass to a multiphase system, the intrinsic modeling approach should be used whenever biomass occupies a significant volume fraction of the culture. A recent report(11) offers the first comparison of intrinsic and nonintrinsic model predictions to actual experimental data gathered from a high-density yeast recycle fermentor. Here, the analysis of Jarzebski et al.(11) has been carried further to show that the improper nonintrinsic model predicts a steady-state culture glucose concentration that differs from that given by the fundamentally correct intrinsic model by over 60% at the optimal, bleed stream flow rate. In addition, a revised formulation for an intrinsic ethanol mass balance is presented.     

Mixing time and oxygen transfer characteristics of double draft tube airlift fermentor

Despite the increasing importance of airlift fermentors, very little published information is available on how the geometric configurations of the draft tubes and the air-sparging system affect the mixing and oxygen transfer characteristics of the fermentor. A 14-L air-lift fermentor was designed and build with a fixed liquid height to diameter ratio of 1.5 utilizing four equally spaced air jets at the bottom. Two jet orifice sizes were used, 1.27 and 3.81 mm i.d., and for each jet size the following four geometric configurations were used: Single inner concentric draft tube, single outer concentric draft tube, two concentric draft tubes, and no draft tubes where the fermentor was operated as a shallow bubble column. It was found that the presence of draft tubes stabilized liquid circulation patterns and gave systemically higher mixing times than those obtained in the absence of draft tubes. In addition, the double draft tube geometry resulted in higher mixing times than the single draft tubes. For the power unit volume range 20 to about 250 W/m³ the larger 3.81-mm orifices gave systemically higher k_L a values than the smaller 1.27-mm i.d. orifices. At 200 W/m³ the use of a single outer draft tube with the 3.81-mm orifices resulted in 94% increase in k_L a values over that obtained with no draft tubes. However, the effect of draft tube geometry on k_L a values when the 1.27-mm orifices were used was not significant. The air bubble formation characteristics at the jet orifices were found to be different, which reflected the differences observed in mass transfer and mixing characteristics. The power economy for oxygen transfer was found to be depend strongly on the orifice size and less on the geometric configuration of draft tubes.     

Modelling mass transfer and agitator performance in multiturbine fermentors

A methodology for mathematically analyzing agitator performance and mass transfer in large multiturbine production fermentors is presented. The application of this approach provides a method for determining axial dissolved oxygen profiles under conditions of known mass transfer rates as a function of agitation-aeration characteristics. A stagewise approach is used which divides the fermentor into a series of mixing cells. This allows for each turbine and mixing cell to be individually optimized. The model also permits the determination of the mass transfer coefficient for each turbine based upon limited dissolved oxygen data. The primary limitation of this approach rests in the limited data and correlations available for multiturbine systems. The structure of the modelling approach can serve as a basis for testing single turbine correlations and adapting them to multiturbine systems. The step-by-step details of the mathematical analysis are presented and interpreted. A series of computer simulations demonstrate the effect of typical fermentor operating variables on the axial dissolved oxygen profile. Further simulations demonstrate the effect of modifying agitator blade numbers on the dissolved oxygen profile and agitator power requirement.     

Kinetics of biodegradation of p-xylene and naphthalene and oxygen transfer in a novel airlift immobilized bioreactor

The scope of this study included the biodegradation performance and the rate of oxygen transfer in a pilot-scale immobilized soil bioreactor system (ISBR) of 10-L working volume. The ISBR was inoculated with an acclimatized population of contaminant degrading microorganisms. Immobilization of microorganisms on a non-woven polyester textile developed the active biofilm, thereby obtaining biodegradation rates of 81 mg/L x h and 40 mg/L x h for p-xylene and naphthalene, respectively. Monod kinetic model was found to be suitable to correlate the experimental data obtained during the course of batch and continuous operations. Oxygen uptake and transfer rates were determined during the batch biodegradation process. The dynamic gassing-out method was used to determine the oxygen uptake rate (OUR) and volumetric oxygen mass transfer, K(L) a. The maximum volumetric OUR of 255 mg O(2)/L x h occurred approximately at 720-722 h after inoculation, when the dry weight of biomass concentration was 0.67 g/L.     

The influence of vessel height and top-section size on the hydrodynamic characteristics of airlift fermentors

Fermentations of the yeast Saccharomyces cerevisiae were carried out in a 90 to 250-L working volume concentric tube airlift fermentor. Measurements of liquid circulation velocity, gas hold-up, and liquid mixing were made under varying conditions of gas flowrate, vessel height, and top-section size. Both liquid circulation velocity and mixing time increased with vessel height. Liquid velocity varied approximately in proportion to the square root of column height, supporting a theoretically based relationship. The effect of vessel height on gas hold-up was negligible. The height of the top-section had a significant effect on liquid mixing. Mixing time decreased with increasing size of the top-section up to a critical height. As the top-section was expanded beyond this height, little improvement in mixing was seen. This indicated the presence of a two-zone flow pattern in the top-section. Liquid velocity and gas hold-up were essentially independent of top-section height. (c) 1994 John Wiley & Sons, Inc.     

Simple system for controlling dissolved oxygen concentration in laboratory fermentors

A simple system for the control of oxygen transfer in laboratory fermentation units has been devised. This system employs pure oxygen to elevate the driving force for oxygen transfer. In addition, the aeration is controlled in a closed loop manner in order to minimize total exit flow of the gas from the fermentor. Fermentation study using Bacillus stearothermophilus grown on n-dodecane at 55°C illustrates excellent carbon recovery as well as the ease of controlling oxygen transfer.     

Low-pressure airlift fermenter for single cell protein production: I. Design and oxygen transfer studies

The energy consumption of a fermenter constitutes a major part of the operating expense of a single cell protein process. A low-pressure airlift fermenter was designed to reduce this cost. In this new design, the fermenter broth is kept below 120 cm in depth, and air alone is employed to fulfill the need of supplying oxygen, and cooling and agitating the broth. The use of low-pressure air from air blowers instead of air compressors lowers the capital cost of air delivery and reduces the energy consumption in the fermenter section to below 1 kWh/kg protein, a saving of over 70% as compared to a conventional stirred tank fermenter. It also eliminates the investment of mechanical agitators, heat exchangers, and air compressors. Sulfite oxidation studies confirmed the design concepts.     

Dynamic modeling and simulation of continuous airlift bioreactors

For dynamic behaviors of continuous airlift bioreactors, a mathematical model based on a tanks-in-series model with backflow has been developed. The equations describing the dynamics of airlift bioreactors are material balances for micro-organism, substrate, dissolved oxygen and oxygen in gas-phase and heat balances. Non-ideal mixing of liquid and gas phases is taken into account using a tanks-in-series model with backflow. The batch operation, startup operation and the consequence of plant failure were simulated and the effects of design and operating parameters for an airlift bioreactor on its dynamic behaviors were discussed. The concentration profiles of micro-organism, substrate, dissolved oxygen and oxygen in gas-phase and the temperature profile in an airlift bioreactors and their dynamics were obtained. The computational results indicate that the transients of a chemostat in the case of bubble column bioreactor are slower compared with those in the case of airlift bioreactor. The proposed simulator is more precise as compared with models published previously in the literature and therefore provides more reliable and rational examination of continuous airlift bioreactor performance.     

Hydrodynamics and mass transfer in a tubular airlift photobioreactor

In photobioreactors, which are usually operated under light limitation,sufficient dissolved inorganic carbon must be provided to avoid carbonlimitation. Efficient mass transfer of CO₂ into the culture mediumisdesirable since undissolved CO₂ is lost by outgassing. Mass transferof O₂ out of the system is also an important consideration, due tothe need to remove photosynthetically-derived O₂ before it reachesinhibitory concentrations. Hydrodynamics (mixing characteristics) are afunctionof reactor geometry and operating conditions (e.g. gas and liquid flow rates),and are a principal determinant of the light regime experienced by the culture.This in turn affects photosynthetic efficiency, productivity, and cellcomposition. This paper describes the mass transfer and hydrodynamics within anear-horizontal tubular photobioreactor. The volume, shape and velocity ofbubbles, gas hold-up, liquid velocity, slip velocity, axial dispersion,Reynoldsnumber, mixing time, and mass transfer coefficients were determined intapwater,seawater, and algal culture medium. Gas hold-up values resembled those ofvertical bubble columns, and the hydraulic regime could be characterized asplug-flow with medium dispersion. The maximum oxygen mass transfer coefficientis approximately 7 h^–1. A regime analysisindicated that there are mass transfer limitations in this type ofphotobioreactor. A methodology is described to determine the mass transfercoefficients for O₂ stripping and CO₂ dissolution whichwould be required to achieve a desired biomass productivity. This procedure canassist in determining design modifications to achieve the desired mass transfercoefficient.     

Oxygen transfer and mixing in mechanically agitated airlift bioreactors

Gas holdup, mixing, liquid circulation and gas–liquid oxygen transfer were characterized in a large (∼1.5 m³) draft-tube airlift bioreactor agitated with Prochem^® hydrofoil impellers placed in the draft-tube. Measurements were made in water and in cellulose fiber slurries that resembled broths of mycelial microfungi. Use of mechanical agitation generally enhanced mixing performance and the oxygen transfer capability relative to when mechanical agitation was not used; however, the oxygen transfer efficiency was reduced by mechanical agitation. The overall volumetric gas–liquid mass transfer coefficient declined with the increasing concentration of the cellulose fiber solids; however, the mixing time in these strongly shear thinning slurries was independent of the solids contents (0–4% w/v). Surface aeration never contributed more than 12% to the total mass transfer in air–water.     

Hydrodynamics and mass transfer in an airlift loop fermentor

Summary The hydrodynamics and mass transfer behaviour of an airlift fermentor with an external loop (height 10m) has been investigated by measuring gas and liquid velocities, gas hold-up, liquid mixing and oxygen transfer coefficients. Liquid phase properties, i.e., ionic strength, viscosity and surface tension have been varied by altering the fermentation media. Results are compared with those from bubble column experiments performed in the same unit. It is shown, that more uniform two-phase flow in the airlift leads to advantages in scale-up and operation.Nomenclature a Specific interfacial area per volume of dispersion (m²/m³) - c Local concentration of tracer (kmol/m³) - c Concentration of tracer at infinite time (kmol/m³) - C_L Concentration of oxygen in the liquid bulk (kmol/m³) - C_L ^* Concentration of oxygen in the interface (kmol/m³) - D_ax Axial dispersion coefficient (cm²/s) - I Ionic strength (kmol/m³) - i Inhomogeneity [defined in Eq. (2)] - Rate of oxygen transfer (kmol/s) - t_c Circulation time (s) - t_M Mixing time (s) - V_R Volume of gas-liquid dispersion (m³) - V_SG Superficial gas velocity in up-flow column (m/s) Greek letter symbols _L Oxygen transfer coefficient (m/s) - Dynamic viscosity (m Pa s) - Surface tension (m N/m) Presented at the First European Congress on Biotechnology, Interlaken, September 25–29, 1978     

Modeling of antibiotics production in magneto three-phase airlift fermenter

A mathematical model is developed to describe the performance of a three-phase airlift reactor utilizing a transverse magnetic field. The model is based on the complete mixing model for the bulk of liquid phase and on the Michaelis-Menten kinetics. The model equations are solved by the explicit finite difference method from transient to steady state conditions. The results of the numerical simulation indicate that the magnetic field increases the degree of bioconversion. The mathematical model is experimentally verified in a three-phase airlift reactor with P. chrysogenum immobilized on magnetic beads. The experimental results are well described by the developed model when the reactor operates in the stabilized regime. At relatively high magnetic field intensities a certain discrepancy in the model solution was observed when the model over estimates the product concentration.